1. Field of the Invention
This invention is related to a monitor, more particularly to a circuit for adjusting a horizontal center of a raster which is developed on a screen of a monitor.
2. Description of the Prior Art
Generally, when manufacturing monitors, there is need for adjusting their deflection circuits to allow the horizontal center of a raster developed on the monitor screen to be aligned with that of the monitor screen before being on the market. Therefore, a raster centering circuit is provided to a deflection circuit of each monitor to allow workers to easily accomplish the adjustment of the raster centering.
FIG. 1 is a view for showing a horizontal deflection circuit that includes a conventional centering circuit for adjusting the horizontal center of a raster.
Referring to FIG. 1, the horizontal deflection circuit includes a horizontal driving circuit 110, a horizontal output circuit 120, a linear correction circuit 130, and a horizontal centering circuit 140.
The horizontal driving circuit 110 generates horizontal driving pulses according to a horizontal frequency signal developed by a horizontal oscillator 100, and outputs the horizontal driving pulses to the horizontal output circuit 120.
The horizontal driving circuit 110 includes a horizontal driving transistor Ql and a horizontal driving transformer (hereinafter, referred to as HDT), as shown in FIG.1. the horizontal driving transistor Q1 is turned on/off in response to the horizontal frequency signal being outputed from the horizontal oscillator 110 to thereby drive a primary coil of the HDT, such that the horizontal driving pulses are generated at a secondary coil of the HDT.
The horizontal output circuit 120 drives the horizontal deflection yoke Hy according to the horizontal driving pulses being outputted from the horizontal driving circuit 110.
The horizontal output circuit 120 includes an output transistor Q2, a damper diode D2, and a resonant capacitor C3.
The base of output transistor Q2 is electrically connected with the secondary coil of the HDT via a resistor R5. The output transistor Q2 is turned on/off by the horizontal driving pulses from the HDT to thereby drive the horizontal deflection yoke Hy. As the output transistor Q2 is turned on/off, a sawtooth current, that is, a horizonal deflection current, which is developed by a contribution of the damper diode D2 and a resonant capacitor C3, flows through the horizontal deflection yoke Hy, as shown in FIG. 2. And, as well known in the art, the horizontal deflection current is corrected by the linear correction circuit 130 and a S-correction capacitor Cs.
Further, a position of a horizontal center of a raster developed on a monitor screen is determined by an amount of the average or direct current, that is, a central level of a ramp of the horizontal deflection current flowing through the horizontal deflection yoke Hy. Therefore, for aligning the horizontal center of the raster with the center of the monitor screen, the DC current flowing through the horizontal deflection yoke Hy must be adjusted.
As shown in FIG. 1, the horizontal centering circuit 140 is connected to a connection node A to which the linear correction circuit 130 and the S correction capacitor Cs are connected with each other. The horizontal centering circuit 140 includes a first switch 141 and a second switch 142, each of which is used to control the amount of a DC current flowing through the horizontal deflection yoke Hy. One terminal of each of the first and second switches 141 and 142 is connected to the connection node A, and each of their other terminals is electrically connected with a B.sup.+ terminal via a first diode D3, a second diode D4, a variable resistor R8, and a choke coil 143 when both the first and second switches 141 and 142 are turned on. When the first switch 141 is turned on and the second switch 142 is turned off, the cathode of the first diode D3 is electrically connected with the connection node A, thereby preventing a current from flowing from the B.sup.+ terminal into the horizontal deflection yoke Hy via the variable resistor R8 and the choke coil 143. To the contrary, when the second switch 142 is turned on and the first switch 141 is turned off, the anode of the second diode D4 is electrically connected with the connection node A, thereby preventing the horizontal deflection current from flowing into the B.sup.+ terminal via the variable resistor R8 and the choke coil 143.
Therefore, achievement results to align the horizontal center of the raster to that of the monitor screen by varying the resistance value of the variable resistor R8 to thereby vary the intensity of DC current flowing through the horizontal deflection yoke Hy after selectively either the first switch 141 or the second switch 142 is turned on.
However, it is difficult and time consuming to precisely align a horizontal center of a raster with the center of a monitor screen by manually adjusting a resistance value of the variable resistor R8. Therefore, it is impossible to improve the productivity of monitor manufacturing with the prior art.
For the foregoing reasons, there is need for providing a circuit which can efficiently adjust a horizontal center of a monitor raster.